[CMake] Fix dropped dependency in install-llvm-headers

This dependency was accidentally dropped in r319480, causing
install-distribution and install-llvm-headers to install an incomplete
set of headers (the generated Intrinsics and Attributes would be
missing).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334452 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed